The present invention relates to a film-grid composite substrate for supporting specimens in electron microscopy. The invention further relates to a strip of such substrates.
Composite substrates for supporting specimens in electron microscopy are well known in the art. Such substrates include a specimen-support grid on which is mounted a very thin specimen-support film. The low penetrating power of electrons in the voltage range typically employed in electron microscopes necessitates, as a practical matter, employment of very thin support films (e.g. 200 to 400 angstroms in thickness) exhibiting high transparency to electrons. In general, thinner films of a given material exhibit greater transparency to electrons. However, the typically lower strength of such thin films necessitates use of film-support grids having small diameter apertures, resulting in greater interference patterns from the correspondingly higher percent grid area occupied by the aperture-defining network. Plastic materials of which heretofore known support films have been formed include Formvar.SM. polyvinyl formal. As described in "Accessories for Microscopy," 1975 Catalog of Ernest F. Fullam, Inc. Schenectady, New York, at page 20, Formvar films are generally on the order of 200 angstroms or more in thickness and have a tendency to decompose in electron beams and to move slightly when first irradiated. Specimen-support films, specimen-support grids and assemblies thereof are described in greater detail by D. E. Bradley in Chapter Three (pages 46-62) ("The Preparation of Specimen Support Films") of Techniques for Electron Microscopy, published by Charles C. Thomas, Springfield, Illinois and edited by Desmond Kay (1961).
Films formed of blends or mixtures of organopolysiloxane-polycarbonate interpolymer and polyphenylene oxide are described in U.S. Pat. No. 3,980,456 (Browall), U.S. Pat. No. 3,874,986 (Browall et al.), and copending U.S. patent application Ser. No. 536,650 filed Dec. 26, 1974 (Ward), assigned to the assignee hereof. The foregoing patents and patent application are incorporated herein by reference. Hereinafter the organopolysiloxane-polycarbonate interpolymer and the polyphenylene oxide polymer are referred to, for brevity, as PSPC and PPO, respectively.
It has now been found by practice of the present invention that ultrathin films of a mixture of PSPC and PPO may be employed in film grid composite substrates for supporting specimens in electron microscopy. Such composite substrates are typically characterized with good adherence of the film to the grid, while the film exhibits improved transparency to electron beams and good stability therein. The film is further characterized with high strength and substantial freedom from contamination and imparts eminent suitability to the composite for use as a specimen-supporting substrate in electron microscopy.
As used herein, the term "transparency" means percent transmission of electrons in electron beams typically employed in electron microscopes. Such beams may have voltages in the range from, e.g. about 50 or less to about 100 or more kilovolts (KV). The term "stability" and terms of like import, as used herein with reference to a characteristic of films in an electron beam, means the extent to which the film resists decomposition and movement during irradiation by the electrons. "Ultrathin" films, as used herein, means films having thickness of not more than about 1,000 angstroms.